A network system typified by LAN (Local Area Network) is constructed of various network devices such as hub. Such network devices are required to address noise for the sake of reliability of communication.
For example, PTL 1 (Japanese Patent Laying-Open No. 9-116510) discloses an interface module with the aim of downsizing the shape and reducing crosstalk. This interface module includes a reception unit and a transmission unit. The reception unit and the transmission unit are each constituted of a low-pass filter, a pulse transformer, and a choke transformer. The low-pass filter is provided for removing a harmonic component (normal-mode noise) and the choke transformer is provided for removing common-mode noise. Further, a shield plate is disposed between the reception unit and the transmission unit.
The above-described network device commonly has a pulse transformer therein. For example, PTL 2 (Japanese Patent Laying-Open No. 61-129806) discloses a structure of the pulse transformer with the aim of suppressing radiation noise, which cannot be suppressed by the pulse transformer, by means of a shield. This pulse transformer includes first and second shield plates that are electrically insulated and arranged opposite to each other with a predetermined distance therebetween, a core member having a predetermined shape and disposed at a predetermined position of the shield plates so that the core member extends through the shield plates, a plurality of windings wound around opposite portions of the core member that are located on the first shield plate and the second shield plate respectively, and a shield member provided to surround a part of the second shield plate. The first shield plate is connected to a case of the device, and the second shield plate is connected via the shield member to an external conductor of a transmission cable having an internal conductor to which one of the windings is connected. Since a signal source includes noise, the pulse transformer is shielded to thereby suppress noise radiation from the cable.
Further, PTL 3 (Japanese Patent Laying-Open No. 2002-270430) for example discloses a structure of the pulse transformer. According to PTL 3, a linear conductor extends through the center of a ring-shaped core to form a primary winding. Around the linear conductor, a plurality of electrostatic shield conductors are arranged. The plurality of electrostatic shield conductors are divided into two groups. A line drawn from one of the groups to the ground is disposed in the opposite direction to a line drawn from the other group to the ground. It is noted that the pulse transformer disclosed in PTL 3 is used in an application for driving the gate of a DC high-voltage switch which is used for a high-voltage pulse generator or the like.
Further, another example of how to address noise is provided for example by PTL 4 (Japanese Patent Laying-Open No. 2004-356070) which discloses a structure for addressing noise of a discharge lamp. More specifically, a part of the discharge lamp socket is covered with a shield cover.
Further, PTL 5 (Japanese Patent Laying-Open No. 63-250199) for example discloses a system for addressing noise of a circuit board on which a transformer is mounted. Specifically, in a region of a substrate surface corresponding to a peripheral portion of the transformer which is a noise source, a pattern is formed in which at least one short-circuit or low-resistance loop is drawn. In this way, the leakage inductance is damped and therefore noise can be lessened.